The Storable package brings persistence to your Perl data structures
containing SCALAR, ARRAY, HASH or REF objects, i.e. anything that can be
conveniently stored to disk and retrieved at a later time.

It can be used in the regular procedural way by calling store with
a reference to the object to be stored, along with the file name where
the image should be written.

The routine returns undef for I/O problems or other internal error,
a true value otherwise. Serious errors are propagated as a die exception.

To retrieve data stored to disk, use retrieve with a file name.
The objects stored into that file are recreated into memory for you,
and a reference to the root object is returned. In case an I/O error
occurs while reading, undef is returned instead. Other serious
errors are propagated via die.

Since storage is performed recursively, you might want to stuff references
to objects that share a lot of common data into a single array or hash
table, and then store that object. That way, when you retrieve back the
whole thing, the objects will continue to share what they originally shared.

At the cost of a slight header overhead, you may store to an already
opened file descriptor using the store_fd routine, and retrieve
from a file via fd_retrieve. Those names aren't imported by default,
so you will have to do that explicitly if you need those routines.
The file descriptor you supply must be already opened, for read
if you're going to retrieve and for write if you wish to store.

You can also store data in network order to allow easy sharing across
multiple platforms, or when storing on a socket known to be remotely
connected. The routines to call have an initial n prefix for network,
as in nstore and nstore_fd. At retrieval time, your data will be
correctly restored so you don't have to know whether you're restoring
from native or network ordered data. Double values are stored stringified
to ensure portability as well, at the slight risk of loosing some precision
in the last decimals.

When using fd_retrieve, objects are retrieved in sequence, one
object (i.e. one recursive tree) per associated store_fd.

If you're more from the object-oriented camp, you can inherit from
Storable and directly store your objects by invoking store as
a method. The fact that the root of the to-be-stored tree is a
blessed reference (i.e. an object) is special-cased so that the
retrieve does not provide a reference to that object but rather the
blessed object reference itself. (Otherwise, you'd get a reference
to that blessed object).

The Storable engine can also store data into a Perl scalar instead, to
later retrieve them. This is mainly used to freeze a complex structure in
some safe compact memory place (where it can possibly be sent to another
process via some IPC, since freezing the structure also serializes it in
effect). Later on, and maybe somewhere else, you can thaw the Perl scalar
out and recreate the original complex structure in memory.

Surprisingly, the routines to be called are named freeze and thaw.
If you wish to send out the frozen scalar to another machine, use
nfreeze instead to get a portable image.

Note that freezing an object structure and immediately thawing it
actually achieves a deep cloning of that structure:

dclone(.) = thaw(freeze(.))

Storable provides you with a dclone interface which does not create
that intermediary scalar but instead freezes the structure in some
internal memory space and then immediately thaws it out.

The lock_store and lock_nstore routine are equivalent to
store and nstore, except that they get an exclusive lock on
the file before writing. Likewise, lock_retrieve does the same
as retrieve, but also gets a shared lock on the file before reading.

As with any advisory locking scheme, the protection only works if you
systematically use lock_store and lock_retrieve. If one side of
your application uses store whilst the other uses lock_retrieve,
you will get no protection at all.

The internal advisory locking is implemented using Perl's flock()
routine. If your system does not support any form of flock(), or if
you share your files across NFS, you might wish to use other forms
of locking by using modules such as LockFile::Simple which lock a
file using a filesystem entry, instead of locking the file descriptor.

Normally, Storable stores elements of hashes in the order they are
stored internally by Perl, i.e. pseudo-randomly. If you set
$Storable::canonical to some TRUE value, Storable will store
hashes with the elements sorted by their key. This allows you to
compare data structures by comparing their frozen representations (or
even the compressed frozen representations), which can be useful for
creating lookup tables for complicated queries.

Canonical order does not imply network order; those are two orthogonal
settings.

Since Storable version 2.05, CODE references may be serialized with
the help of the B::Deparse manpage. To enable this feature, set
$Storable::Deparse to a true value. To enable deserializazion,
$Storable::Eval should be set to a true value. Be aware that
deserialization is done through eval, which is dangerous if the
Storable file contains malicious data. You can set $Storable::Eval
to a subroutine reference which would be used instead of eval. See
below for an example using a the Safe manpage compartment for deserialization
of CODE references.

If $Storable::Deparse and/or $Storable::Eval are set to false
values, then the value of $Storable::forgive_me (see below) is
respected while serializing and deserializing.

This release of Storable can be used on a newer version of Perl to
serialize data which is not supported by earlier Perls. By default,
Storable will attempt to do the right thing, by croak()ing if it
encounters data that it cannot deserialize. However, the defaults
can be changed as follows:

Perl 5.6 added support for Unicode characters with code points > 255,
and Perl 5.8 has full support for Unicode characters in hash keys.
Perl internally encodes strings with these characters using utf8, and
Storable serializes them as utf8. By default, if an older version of
Perl encounters a utf8 value it cannot represent, it will croak().
To change this behaviour so that Storable deserializes utf8 encoded
values as the string of bytes (effectively dropping the is_utf8 flag)
set $Storable::drop_utf8 to some TRUE value. This is a form of
data loss, because with $drop_utf8 true, it becomes impossible to tell
whether the original data was the Unicode string, or a series of bytes
that happen to be valid utf8.

Perl 5.8 adds support for restricted hashes, which have keys
restricted to a given set, and can have values locked to be read only.
By default, when Storable encounters a restricted hash on a perl
that doesn't support them, it will deserialize it as a normal hash,
silently discarding any placeholder keys and leaving the keys and
all values unlocked. To make Storable croak() instead, set
$Storable::downgrade_restricted to a FALSE value. To restore
the default set it back to some TRUE value.

Earlier versions of Storable would immediately croak if they encountered
a file with a higher internal version number than the reading Storable
knew about. Internal version numbers are increased each time new data
types (such as restricted hashes) are added to the vocabulary of the file
format. This meant that a newer Storable module had no way of writing a
file readable by an older Storable, even if the writer didn't store newer
data types.

This version of Storable will defer croaking until it encounters a data
type in the file that it does not recognize. This means that it will
continue to read files generated by newer Storable modules which are careful
in what they write out, making it easier to upgrade Storable modules in a
mixed environment.

The old behaviour of immediate croaking can be re-instated by setting
$Storable::accept_future_minor to some FALSE value.

All these variables have no effect on a newer Perl which supports the
relevant feature.

Storable uses the ``exception'' paradigm, in that it does not try to workaround
failures: if something bad happens, an exception is generated from the
caller's perspective (see the Carp manpage and croak()). Use eval {} to trap
those exceptions.

When Storable croaks, it tries to report the error via the logcroak()
routine from the Log::Agent package, if it is available.

Normal errors are reported by having store() or retrieve() return undef.
Such errors are usually I/O errors (or truncated stream errors at retrieval).

Any class may define hooks that will be called during the serialization
and deserialization process on objects that are instances of that class.
Those hooks can redefine the way serialization is performed (and therefore,
how the symmetrical deserialization should be conducted).

Since we said earlier:

dclone(.) = thaw(freeze(.))

everything we say about hooks should also hold for deep cloning. However,
hooks get to know whether the operation is a mere serialization, or a cloning.

Therefore, when serializing hooks are involved,

dclone(.) <> thaw(freeze(.))

Well, you could keep them in sync, but there's no guarantee it will always
hold on classes somebody else wrote. Besides, there is little to gain in
doing so: a serializing hook could keep only one attribute of an object,
which is probably not what should happen during a deep cloning of that
same object.

The serializing hook, called on the object during serialization. It can be
inherited, or defined in the class itself, like any other method.

Arguments: obj is the object to serialize, cloning is a flag indicating
whether we're in a dclone() or a regular serialization via store() or freeze().

Returned value: A LIST ($serialized, $ref1, $ref2, ...) where $serialized
is the serialized form to be used, and the optional $ref1, $ref2, etc... are
extra references that you wish to let the Storable engine serialize.

At deserialization time, you will be given back the same LIST, but all the
extra references will be pointing into the deserialized structure.

The first time the hook is hit in a serialization flow, you may have it
return an empty list. That will signal the Storable engine to further
discard that hook for this class and to therefore revert to the default
serialization of the underlying Perl data. The hook will again be normally
processed in the next serialization.

The deserializing hook called on the object during deserialization.
But wait: if we're deserializing, there's no object yet... right?

Wrong: the Storable engine creates an empty one for you. If you know Eiffel,
you can view STORABLE_thaw as an alternate creation routine.

This means the hook can be inherited like any other method, and that
obj is your blessed reference for this particular instance.

The other arguments should look familiar if you know STORABLE_freeze:
cloning is true when we're part of a deep clone operation, serialized
is the serialized string you returned to the engine in STORABLE_freeze,
and there may be an optional list of references, in the same order you gave
them at serialization time, pointing to the deserialized objects (which
have been processed courtesy of the Storable engine).

When the Storable engine does not find any STORABLE_thaw hook routine,
it tries to load the class by requiring the package dynamically (using
the blessed package name), and then re-attempts the lookup. If at that
time the hook cannot be located, the engine croaks. Note that this mechanism
will fail if you define several classes in the same file, but the perlmod manpage
warned you.

It is up to you to use this information to populate obj the way you want.

While STORABLE_freeze and STORABLE_thaw are useful for classes where
each instance is independant, this mechanism has difficulty (or is
incompatible) with objects that exist as common process-level or
system-level resources, such as singleton objects, database pools, caches
or memoized objects.

Arguments: class is the class we are attaching to, cloning is a flag
indicating whether we're in a dclone() or a regular de-serialization via
thaw(), and serialized is the stored string for the resource object.

Because these resource objects are considered to be owned by the entire
process/system, and not the ``property'' of whatever is being serialized,
no references underneath the object should be included in the serialized
string. Thus, in any class that implements STORABLE_attach, the
STORABLE_freeze method cannot return any references, and Storable
will throw an error if STORABLE_freeze tries to return references.

All information required to ``attach'' back to the shared resource object
must be contained only in the STORABLE_freeze return string.
Otherwise, STORABLE_freeze behaves as normal for STORABLE_attach
classes.

Because STORABLE_attach is passed the class (rather than an object),
it also returns the object directly, rather than modifying the passed
object.

With hooks comes the ability to recurse back to the Storable engine.
Indeed, hooks are regular Perl code, and Storable is convenient when
it comes to serializing and deserializing things, so why not use it
to handle the serialization string?

There are a few things you need to know, however:

You can create endless loops if the things you serialize via freeze()
(for instance) point back to the object we're trying to serialize in
the hook.

Shared references among objects will not stay shared: if we're serializing
the list of object [A, C] where both object A and C refer to the SAME object
B, and if there is a serializing hook in A that says freeze(B), then when
deserializing, we'll get [A', C'] where A' refers to B', but C' refers to D,
a deep clone of B'. The topology was not preserved.

That's why STORABLE_freeze lets you provide a list of references
to serialize. The engine guarantees that those will be serialized in the
same context as the other objects, and therefore that shared objects will
stay shared.

In the above [A, C] example, the STORABLE_freeze hook could return:

("something", $self->{B})

and the B part would be serialized by the engine. In STORABLE_thaw, you
would get back the reference to the B' object, deserialized for you.

Therefore, recursion should normally be avoided, but is nonetheless supported.

There is a Clone module available on CPAN which implements deep cloning
natively, i.e. without freezing to memory and thawing the result. It is
aimed to replace Storable's dclone() some day. However, it does not currently
support Storable hooks to redefine the way deep cloning is performed.

Yes, there's a lot of that :-) But more precisely, in UNIX systems
there's a utility called file, which recognizes data files based on
their contents (usually their first few bytes). For this to work,
a certain file called magic needs to taught about the signature
of the data. Where that configuration file lives depends on the UNIX
flavour; often it's something like /usr/share/misc/magic or
/etc/magic. Your system administrator needs to do the updating of
the magic file. The necessary signature information is output to
STDOUT by invoking Storable::show_file_magic(). Note that the GNU
implementation of the file utility, version 3.38 or later,
is expected to contain support for recognising Storable files
out-of-the-box, in addition to other kinds of Perl files.

If you're using references as keys within your hash tables, you're bound
to be disappointed when retrieving your data. Indeed, Perl stringifies
references used as hash table keys. If you later wish to access the
items via another reference stringification (i.e. using the same
reference that was used for the key originally to record the value into
the hash table), it will work because both references stringify to the
same string.

It won't work across a sequence of store and retrieve operations,
however, because the addresses in the retrieved objects, which are
part of the stringified references, will probably differ from the
original addresses. The topology of your structure is preserved,
but not hidden semantics like those.

On platforms where it matters, be sure to call binmode() on the
descriptors that you pass to Storable functions.

Storing data canonically that contains large hashes can be
significantly slower than storing the same data normally, as
temporary arrays to hold the keys for each hash have to be allocated,
populated, sorted and freed. Some tests have shown a halving of the
speed of storing -- the exact penalty will depend on the complexity of
your data. There is no slowdown on retrieval.

You can't store GLOB, FORMLINE, etc.... If you can define semantics
for those operations, feel free to enhance Storable so that it can
deal with them.

The store functions will croak if they run into such references
unless you set $Storable::forgive_me to some TRUE value. In that
case, the fatal message is turned in a warning and some
meaningless string is stored instead.

Setting $Storable::canonical may not yield frozen strings that
compare equal due to possible stringification of numbers. When the
string version of a scalar exists, it is the form stored; therefore,
if you happen to use your numbers as strings between two freezing
operations on the same data structures, you will get different
results.

When storing doubles in network order, their value is stored as text.
However, you should also not expect non-numeric floating-point values
such as infinity and ``not a number'' to pass successfully through a
nstore()/retrieve() pair.

As Storable neither knows nor cares about character sets (although it
does know that characters may be more than eight bits wide), any difference
in the interpretation of character codes between a host and a target
system is your problem. In particular, if host and target use different
code points to represent the characters used in the text representation
of floating-point numbers, you will not be able be able to exchange
floating-point data, even with nstore().

Storable::drop_utf8 is a blunt tool. There is no facility either to
return all strings as utf8 sequences, or to attempt to convert utf8
data back to 8 bit and croak() if the conversion fails.

Prior to Storable 2.01, no distinction was made between signed and
unsigned integers on storing. By default Storable prefers to store a
scalars string representation (if it has one) so this would only cause
problems when storing large unsigned integers that had never been coverted
to string or floating point. In other words values that had been generated
by integer operations such as logic ops and then not used in any string or
arithmetic context before storing.

This section only applies to you if you have existing data written out
by Storable 2.02 or earlier on perl 5.6.0 or 5.6.1 on Unix or Linux which
has been configured with 64 bit integer support (not the default)
If you got a precompiled perl, rather than running Configure to build
your own perl from source, then it almost certainly does not affect you,
and you can stop reading now (unless you're curious). If you're using perl
on Windows it does not affect you.

Storable writes a file header which contains the sizes of various C
language types for the C compiler that built Storable (when not writing in
network order), and will refuse to load files written by a Storable not
on the same (or compatible) architecture. This check and a check on
machine byteorder is needed because the size of various fields in the file
are given by the sizes of the C language types, and so files written on
different architectures are incompatible. This is done for increased speed.
(When writing in network order, all fields are written out as standard
lengths, which allows full interworking, but takes longer to read and write)

Perl 5.6.x introduced the ability to optional configure the perl interpreter
to use C's long long type to allow scalars to store 64 bit integers on 32
bit systems. However, due to the way the Perl configuration system
generated the C configuration files on non-Windows platforms, and the way
Storable generates its header, nothing in the Storable file header reflected
whether the perl writing was using 32 or 64 bit integers, despite the fact
that Storable was storing some data differently in the file. Hence Storable
running on perl with 64 bit integers will read the header from a file
written by a 32 bit perl, not realise that the data is actually in a subtly
incompatible format, and then go horribly wrong (possibly crashing) if it
encountered a stored integer. This is a design failure.

Storable has now been changed to write out and read in a file header with
information about the size of integers. It's impossible to detect whether
an old file being read in was written with 32 or 64 bit integers (they have
the same header) so it's impossible to automatically switch to a correct
backwards compatibility mode. Hence this Storable defaults to the new,
correct behaviour.

What this means is that if you have data written by Storable 1.x running
on perl 5.6.0 or 5.6.1 configured with 64 bit integers on Unix or Linux
then by default this Storable will refuse to read it, giving the error
Byte order is not compatible. If you have such data then you you
should set $Storable::interwork_56_64bit to a true value to make this
Storable read and write files with the old header. You should also
migrate your data, or any older perl you are communicating with, to this
current version of Storable.

If you don't have data written with specific configuration of perl described
above, then you do not and should not do anything. Don't set the flag -
not only will Storable on an identically configured perl refuse to load them,
but Storable a differently configured perl will load them believing them
to be correct for it, and then may well fail or crash part way through
reading them.

Benjamin Holzman contributed the tied variable support, Andrew Ford
contributed the canonical order for hashes, and Gisle Aas fixed
a few misunderstandings of mine regarding the perl internals,
and optimized the emission of ``tags'' in the output streams by
simply counting the objects instead of tagging them (leading to
a binary incompatibility for the Storable image starting at version
0.6--older images are, of course, still properly understood).
Murray Nesbitt made Storable thread-safe. Marc Lehmann added overloading
and references to tied items support.

Please e-mail us with problems, bug fixes, comments and complaints,
although if you have complements you should send them to Raphael.
Please don't e-mail Raphael with problems, as he no longer works on
Storable, and your message will be delayed while he forwards it to us.